collision detection

Carrier sense multiple access with collision detection

Carrier Sense Multiple Access With Collision Detection (CSMA/CD), in computer networking, is a network control protocol in which

CSMA/CD is a modification of pure Carrier Sense Multiple Access (CSMA).

Collision detection is used to improve CSMA performance by terminating transmission as soon as a collision is detected, and reducing the probability of a second collision on retry.

Methods for collision detection are media dependent, but on an electrical bus such as Ethernet, collisions can be detected by comparing transmitted data with received data. If they differ, another transmitter is overlaying the first transmitter's signal (a collision), and transmission terminates immediately. A jam signal is sent which will cause all transmitters to back off by random intervals, reducing the probability of a collision when the first retry is attempted. CSMA/CD is a layer 2 protocol in the OSI model.

Ethernet is the classic CSMA/CD protocol. However, CSMA/CD is no longer used in the 10 Gigabit Ethernet specifications, due to the requirement of switches replacing all hubs and repeaters. Similarly, while CSMA/CD operation (half duplex) is defined in the Gigabit Ethernet specifications, few implementations support it and in practice it is nonexistent. Also, in Full Duplex Ethernet, collisions are impossible since data is transmitted and received on different wires, and each segment is connected directly to a switch. Therefore, CSMA/CD is not used on Full Duplex Ethernet networks.

Very partly derived from Federal Standard 1037C

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